The present invention relates to control equipment for thyristor-controlled series-capacitor equipment, which is intended for connection between an ac network and a converter, connected thereto, for conversion between alternating voltage and high-voltage direct current, and to a method for control of such series-capacitor equipment.
It is known to series-compensate converters of the kind described above by connecting the valve bridges of the converter to the ac network via series capacitors. This entails several advantages. The series capacitors are charged periodically by the current traversing them, and the voltages which are thereby generated across the capacitors constitute additions to the commutating voltages across the valves of the converter. The commutating voltages are phase-shifted related to the voltages of the ac network such that, with the control and extinction angles of the converter related to the voltages of the ac network, the valves during rectifier operation may be controlled with control angles smaller than zero and in inverter operation with extinction angles smaller than zero. In this way, as viewed from the ac network, a reduction of the consumption of reactive power by the converter is achieved, and by suitable dimensioning of the capacitors, the dependence of the overlap angle on the magnitude of the direct current may be compensated. This means that the series compensation contributes to maintain the margin of commutation of the valves also in case of rapid current transients.
FIG. 1 illustrates such a known configuration. The figure will be described in greater detail below with reference to the description of embodiments of the invention.
I Woodford and Zheng, xe2x80x9cSeries Compensation of DC Linksxe2x80x9d. Power electronics in electric power systemsxe2x80x94Symposium; International conference on large high voltage electric systems, Cigre, Tokyo May 1995, a configuration is described where the series capacitor is connected between the ac network and the converter in such a way that the ac filters belonging to the converter are placed between the transformer of the converter and the series capacitor equipment. The document refers to previous investigations in which ferroresonance, that is, oscillations in the subsynchronous frequency range between the series capacitor and the magnetic circuit of the converter transformer and at which the latter becomes saturated, is identified as a problem in similar configurations. The document states temporary connection of an inductor in parallel with the series capacitor to be an efficient method for damping such oscillations. Further, it is suggested that the series capacitor, essentially for economic reasons, be divided into a plurality of series-connected sections, which may each in a known way be bypassed or connected. FIG. 2A illustrates schematically, in the form of a single-line diagram, one such known configuration. The series capacitor comprises a number of sections C1, C2, C3, each one being connected in parallel with a fixed inductor, L11, L12, L13, respectively, and an inductor, L21, L22, L23, respectively, which may be temporarily connected by means of a switching member S21, S22, S23, respectively. Switching members S11, S12, S13 make possible connection and disconnection of individual sections. The document further mentions, in general terms, that additional damping is possible by controlling the current through the parallel-connected inductor by means of a thyristor switch comprising two thyristors in anti-parallel connection.
U.S. Pat. No. 5,032,738 (Vithayathil, J. H.) describes, in particular in FIG. 3A of the patent, a device connected in a transmission line and comprising a series capacitor connected in parallel with an inductor. The current through the inductor is controlled by a thyristor switch connected in series with the inductor and comprising two thyristors in anti-parallel connection. The purpose of the device is stated to be to achieve fast and continuous changes of the transmission impedance of the transmission line and to facilitate damping of subsynchronous oscillations as well as to modulate the current in the transmission line.
A device of this kind is usually referred to as a thyristor-controlled series capacitor, or a TCSC, and is schematically illustrated in the form of a single-line diagram in FIG. 2B. A capacitor C is connected in parallel with an inductor L and a controllable semiconductor valve TYSW, the last-mentioned components being mutually series-connected. The semiconductor valve comprises two thyristors, TY1 and TY2, respectively, in anti-parallel connection. The semiconductor is supplied with firing pulses TP1, TP2 which bring the respective thyristors to the conducting state at firing instants determined by control equipment (not shown).
The published European patent application EP 0 689 272 A1, which is hereby included as a reference, describes control members for controlling the firing instants for the semiconductor valve in a TCSC. The control members comprise a command signal-generating member, preferably a phase-locked loop synchronized to the current through the transmission line, which phase-locked loop delivers equidistant command signals which are substantially independent of subsynchronous components and which are used as reference times for initiation of firing pulses for the semiconductor valve. A calculating circuit is adapted, based on these reference times in dependence on the instantaneous values of the line current and the capacitor voltage, to determine the firing instants in such a way that the instants for the zero crossings of the capacitor voltage occur with a constant time delay relative to the reference times. The zero crossings thus become substantially equidistant also in the presence of subsynchronous components in the current through the transmission line. The method comprises damping or completely preventing subsynchronous resonant oscillations, independently of a superordinate control of the effective impedance of the series capacitor at the fundamental frequency.
The firing instants for the semiconductor valve must be synchronized with respect to either the voltage across or the current through the capacitor. This is done conventionally by bringing a phase-locked loop to generate a signal related to the phase position for the fundamental tone of one of these quantities.
Studies in this respect have shown that synchronization to the current through the capacitor provides better dynamic properties for the thyristor-controlled series capacitor.
FIGS. 3 and 4, which will be described in greater detail below with reference to the description of embodiments of the invention, schematically illustrate, respectively, known control, equipment for a TCSC and typical signal shapes in such control equipment.
A phase-locked loop usually comprises a low-pass filter which limits its bandwidth, that is, its ability to rapidly reproduce a change in the phase position of the current. Typically, this bandwidth is in the interval of 10-40 Hz.
Thyristor-controlled series capacitor equipment, which is connected between an ac network and a converter connected to the ac network, is subjected to rapid transients in the phase position of the current, in particular during operational disturbances such as, for example, when the converter temporarily loses its load and thereafter resumes it. Such a situation may, for example, arise when the converter is blocked in connection with a line fault and then, when the fault has been cleared, resumes operation as soon as possible. In that context, a situation may arise whereby the phase-locked loop does not provide the correct information about the present phase position of the current, which leads to the semiconductor valve being fired at unsuitable firing instants, resulting in large and possibly harmful valve currents.
The object of the invention is to provide control equipment of the kind described in the introductory part of the description, which is improved with respect to its ability for synchronization with the phase position of the current through the series capacitor equipment, in particular when this phase position undergoes rapid changes, and a method for control of the series capacitor equipment.
The dynamic performance of the thyristor-controlled series capacitor depends to a great extent on whether reliable and rapid information about the phase position of the applied current is available. Such information is available at the earliest in the control device of the converter as a measure of the control angle thereof.
A control device forms a control-angle order for phase-angle control of the converter, related to the phase position for a voltage at a chosen connection point, and the object of the invention is achieved in that the control equipment, which comprises control means which in dependence on a synchronization signal forms firing pulses for control of the semiconductor valve, comprises phase-correcting means forming the synchronization signal in dependence on a voltage-phase signal applied thereto, which contains information about the phase position for the voltage in question, and on a control-angle signal supplied thereto, which contains information about the control angle of the converter.
According to a further development of the invention, the control equipment forms the synchronization signal in dependence also on a dc signal supplied thereto, which contains information about the amplitude for a sensed direct current through the converter.
According to another further development of the invention, the control means form the firing pulses for control of the semiconductor valve in dependence on an ac signal, containing information about the instantaneous amplitude of a sensed alternating current flowing through the series capacitor equipment, and on the ac voltage signal, containing information about the instantaneous amplitude for a sensed ac voltage across the series capacitor equipment.
According to yet another further development of the invention, in which the control equipment comprises a controller for control of the fundamental-tone reactance of the series capacitor equipment in dependence on a difference of a reference value for the fundamental-tone reactance and an actual value thereof, supplied to the controller, the control equipment comprises reference-value forming means which form the reference value in dependence on a dc signal supplied to the control equipment and containing information about the amplitude for a sensed direct current through the converter.